Process for organosolv pulping and use of a gamma lactone in a solvent for organosolv pulping

ABSTRACT

The invention provides a process for organosolv pulping, wherein solid lignocellulosic feed material is heated at a temperature in the range of from 50 to 210° C. in a solvent to obtain a solid cellulosic fraction comprising at least 50 wt % of the cellulose present in the feed material and a liquid fraction, wherein the solvent comprises at least 10 wt % of a compound according to general molecular formula wherein R&lt;SUB&gt;1 &lt;/SUB&gt;to R&lt;SUB&gt;6 &lt;/SUB&gt;each represent, independently, a hydrogen atom or an organic group connected with a carbon atom to the lactone group. The invention further provides the use of a compound according to general molecular formula (1) in a solvent for organosolv pulping.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from European Patent Application No. 05105245.4, filed Jun. 15, 2005, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention provides a process for organosolv pulping and the use of a gamma lactone in a solvent for organosolv pulping.

BACKGROUND OF THE INVENTION

The most widely used pulp manufacturing process is the Kraft process. An important drawback of the Kraft pulping process is, however, that a large mineral waste stream comprising harmful components is formed. In order to avoid the formation of mineral waste material, organosolv pulping has been proposed and studied as an alternative for Kraft pulping. In organosolv pulping, lignocellulosic material is heated in a solvent comprising organic compounds and optionally water, in order to dissolve the greater part of the hemicellulose and lignin and to obtain a high-quality, high-molecular weight cellulose that is suitable for paper production. The solvent is separated from the dissolved hemicellulose and lignin by simple distillation for recycling.

Well-known solvents for organosolv pulping comprise organic compounds such as lower aliphatic alcohols, for example methanol or ethanol, lower carboxylic acids, for example formic acid or acetic acid, acetone, polyhydric alcohols, for example ethylene glycol or glycerol, or mixtures thereof. Often water is part of the solvent, typically in an amount up to 50 wt %. A small amount of strong mineral acid, typically in the range of a few tenths to a few percent, may be added as catalyst to the solvent. Oxidants such as hydrogen peroxide or peroxy acids may be added to the solvent to improve bleaching.

Organosolv pulping is typically carried out at a temperature in the range of from 80 to 180° C. The operating pressure mainly depends on the volatility of the solvent. The pressure should be such that the solvent is still in the liquid phase. Typically, organolsolv pulping is performed just below the boiling temperature of the liquor. A drawback of the use of relatively volatile solvents is therefore that relatively high operating pressures are needed.

An extensive overview of prior art organosolv processes is given in E. Muurinen, “Organosolv Pulping—A review and distillation study related to peroxyacid pulping”, University of Oulu, Finland, 2000, ISBN 951-42-5661-1.

SUMMARY OF THE INVENTION

It has now been found that compounds having a gamma lactone group can very suitably be used as solvent or part of the solvent for organosolv pulping.

Accordingly, the present invention provides a process for organosolv pulping, wherein solid lignocellulosic feed material is heated at a temperature in the range of from 50 to 210° C. in a solvent to obtain a solid cellulosic fraction comprising at least 50 wt % of the cellulose present in the feed material and a liquid fraction, wherein the solvent comprises at least 10 wt % of a compound according to general molecular formula

wherein R₁ to R₆ each represent, independently, a hydrogen atom or an organic group connected with a carbon atom to the lactone group.

In a further aspect, the invention provides the use of a compound according to general molecular formula (1) in a solvent for organosolv pulping.

An important advantage of the use of a compound having a gamma lactone group in a solvent for organosolv pulping is that it has a relatively high boiling point and that the organosolv pulping can thus be carried out at a relatively low pressure.

A further advantage of the process and the use according to the invention is that the compound according to general molecular formula (1) is both polar and relatively inert. As a consequence, it is an effective organosolv solvent that does hardly form reaction products with the lignocellulosic feed material or with components formed during the organosolv process.

DETAILED DESCRIPTION OF THE INVENTION

The process according to the invention is a process for organosolv pulping wherein a solid lignocellulosic feed material is heated in a solvent comprising at least 10 wt % of a compound having a gamma lactone group to obtain a solid cellulosic fraction and a liquid fraction. The liquid fraction contains the solvent and dissolved hemicellulose and lignin degradation products. The aim of organosolv pulping is to obtain a high-quality solid cellulose fraction or cellulose pulp that is suitable for paper production. Therefore, the exact process conditions in terms of temperature, pressure, heating time and the solvent used are chosen such that the greater part of the cellulose remains intact, i.e. is not depolymerised and dissolved in the liquid fraction. In the process according to the invention at least 50 wt % of the cellulose present in the feed material is recovered in the solid cellulosic fraction, preferably at least 60 wt %, more preferably at least 80 wt %. The cellulose content of the feed material and of the solid cellulose fraction obtained may for example be determined by hydrolysing a sample of the material followed by identification and quantification of sugars by means of gas chromatography according to TAPPI method T 249 cm-00. After correction of the glucose value by subtracting the glucose portion derived from the glucomannan present in the sample, the cellulose content is calculated from the corrected glucose value.

Preferably, the solid cellulosic fraction obtained has an average degree of polymerisation of at least 300. Reference herein to the average degree of polymerisation of the cellulose is to the weight-average degree of polymerisation. The degree of polymerisation may be determined by measuring the viscosity of a solution of the cellulosic fraction of known concentration, for example according to TAPPI method T 230 om-04.

Preferably, at least 50 wt % of the lignin in the lignocellulosic feed material is removed from the feed material during the organosolv process according to the invention, more preferably at least 80 wt %.

The lignocellulosic feed material is heated in the solvent at a temperature that is typical for organosolv processes, i.e. in the range of from 50 to 210° C., preferably of from 100 to 200° C., more preferably of from 80 to 180° C.

The solvent used in the process according to the invention comprises at least 10 wt % of a compound having a gamma lactone group, i.e. a compound according to general molecular formula (1). Preferably, the solvent comprises at least 20 wt % of such compound, more preferably at least 50 wt %, even more preferably at least 80 wt %, based on the total weight of the solvent.

Reference herein to the solvent is to the total liquid phase in which the solid feed material is heated. Apart from one or more compounds according to general molecular formula (1), the solvent may comprise further organic compounds that are known solvents for organosolv pulping. Examples of such known compounds are lower aliphatic alcohols such as methanol or ethanol, polyhydric alcohols, in particular diols with the hydroxyl groups on adjacent carbon atoms such as ethylene glycol, glycerol, 1,2-propanediol or 2,3-butanediol, lower carboxylic acids such as formic acid or acetic acid, and acetone. The solvent may also comprise water, preferably in an amount up to 50 wt %, more preferably up to 20 wt %.

The solvent may comprise an acid catalyst. Any acid known to be suitable as catalyst in organosolv pulping may be used. In particular strong mineral acids such as phosphoric acid, sulphuric acid, hydrochloric acid and nitric acid, are known to be very effective catalysts for organosolv pulping. The catalyst preferably is a strong mineral or organic acid with a pKa below 2.5. Preferred strong mineral acids are phosphoric acid and sulphuric acid, more preferably phosphoric acid. Preferred strong organic acids are oxalic acid, 2-oxopropanoic acid, maleic acid, and 2,4,6-trihydroxibenzoic acid. Combinations of acids may also be used.

It is preferred to keep the concentration of mineral compounds in the solvent as low as possible in order to avoid mineral waste streams. The acid catalyst, in particular in case of a mineral catalyst, is therefore preferably present in a concentration of below 5 wt % of the solvent, more preferably in a concentration in the range of from 0.01 to 3.0 wt %, even more preferably of from 0.05 to 1.0 wt %. For a heating temperature in the range of from 100 to 210° C., it is preferred to use a solvent that is free of mineral acid. Therefore, in a preferred embodiment of the process according to the invention, the feed material is heated at a temperature in the range of from 100 to 210° C., more preferably of from 120 to 180° C., in a solvent that is free of mineral acid. For lower heating temperatures, i.e. below 100° C., the presence of an acid catalyst in the solvent is preferred.

The lignocellulosic feed material may be any lignocellulosic material known to be a suitable feedstock for pulping processes. Examples of such materials are hardwood, softwood, bagasse, wheat straw, miscanthus, switch grass, reed, or flax. The feed material may be in any form known to be suitable for organosolv pulping, typically in the form of particles with dimensions in the order of a few centimetres, for example wood chips or cutted stalks.

The organosolv process according to the invention may be carried out in a batch, semi-batch or continuous operation. In a batch operation, the ratio of solvent to solid feed material is preferably in the range of from 2 to 50, more preferably of from 3 to 15. In a continuous operation, i.e. with continuous supply and discharge of solvent, the liquid hourly velocity of the solvent is preferably in the range of from 1 to 50 litre solvent per kg feed material per hour, more preferably of from 2 to 25 litre/kg/h.

Reference herein to a compound having a gamma lactone group is to a compound according to general molecular formula (1), wherein R₁ to R₆ each represent, independently, a H atom or an organic group connected with a carbon atom to the lactone group. The total number of carbon atoms of the compound is preferably at most 20, more preferably at most 15.

The compound according to general molecular formula (1) preferably is a compound wherein R₁, R₂, R₃ and R₄ are a hydrogen atom, more preferably a compound wherein R₁, R₂, R₃ and R₄ are a hydrogen atom and R₅ is a methyl group. Examples of such more preferred compounds are gamma valerolactone (R₆ is a hydrogen atom) also known as 5-methyldihydrofuran-2(3H)-one, 2-methyl-5-oxotetrahydrofuran-2-carboxylic acid (R₆ is a carboxyl group), a compound having a molecular structure according to any one of molecular formulas (2) to (5):

or an ester of a compound having a molecular structure according to molecular formula (2) or (3).

The compounds according to formulas (2) to (5) are levulinic acid dimers that may be obtained by contacting levulinic acid in the presence of hydrogen with a strongly acidic catalyst having a hydrogenating function, e.g. Pd/cation-exchange resin, at elevated temperature and preferably at elevated pressure. Typical process temperatures and pressures are in the range of from 60 to 170° C. and of from 1 to 200 bar (absolute), respectively. Such process for levulinic acid dimerisation is described in detail in co-pending patent application EP 04106107.8. The catalyst and process conditions of this process are similar to those applied in the known single-step process for the production of methyl isobutyl ketone from acetone.

Other compounds with a gamma lactone group suitable to be used in the solvent of the organosolv process according to the invention, which are obtainable by the above-mentioned levulinic acid dimerisation process, are the compounds with a molecular structure according to formula (6) or (7) or their esters:

If the compound with a gamma lactone group is an ester of an acid according to molecular formula (2), (3), (6) or (7), then the ester preferably is an alkyl ester with an alcohol fragment with at most 10 carbon atoms, more preferably a linear alkyl ester with an alcohol fragment with at most 5 carbon atoms, even more preferably a methyl or an ethyl ester.

In the organosolv process according to the invention, the feed material is preferably heated in the solvent at a pressure in the range of from 1 to 10 bar (absolute), more preferably of from 1 to 5 bar (absolute). Since compounds according to general formula (1) have a relatively low volatility as compared to conventional organosolv solvents like lower aliphatic carboxylic acids or alcohols, the organosolv process according to the invention can be carried out at relatively low pressures.

Preferably, the compound(s) according to general molecular formula (1) that are used in the solvent are recovered for recycling. This may for example be done by applying the following process steps, after the heating of the feed material in the solvent as hereinabove described:

-   (a) separating the solid cellulose fraction from the liquid     fraction; -   (b) adding water to the separated liquid fraction and heating the     separated liquid fraction in the presence of an acid catalyst at a     temperature in the range of from 100 to 300° C. to obtain a     hydrolysed liquid fraction; -   (c) distilling lower boiling compounds in the hydrolysed liquid     fraction from the compound according to general molecular formula     (1); and -   (d) using the compound according to general molecular formula (1)     obtained in step (d) in the solvent wherein the feed material is     heated.

In step (a), the solid and liquid fraction obtained in the organosolv process according to the invention are separated from each other by conventional means, e.g. filtration. The thus-obtained liquid fraction comprises solvent and dissolved feed material degradation products, mainly depolymerised hemicellulose and lignin. In step (b), the liquid fraction is further hydrolysed after addition of water in order to convert the dissolved feed material degradation products into compounds that boil at a lower temperature than the compound according to general molecular formula (1). Hydrolysis step (b) is carried out in the presence of an acid catalyst. In case the liquid fraction already contained an acid catalyst, i.e. the acid catalyst used in the organosolv step, no additional catalyst needs to be added. In case the organosolv step has been carried out without an acid catalyst, an homogeneous or heterogeneous acid catalyst, preferably a heterogeneous acid catalyst, has to be added to the liquid fraction obtained in step (a).

In step (c), the hydrolysed dissolved feed material degradation products are distilled from the compound(s) according to general molecular formula (1). If lower boiling conventional solvent compounds were present in the solvent, for example methanol, ethanol, formic acid or acetic acid, they will also be distilled from the compound(s) according to general molecular formula (1). If, however, high boiling conventional compounds for organosolv solvents are present in the solvent, i.e. compounds with a comparable or higher boiling point than the compound according to formula (1), they will be retained in the bottom fraction, together with the compound according to general molecular formula (1).

In step (d), the compound according to general molecular formula (1) is recycled to the organosolv step to be used in the solvent.

It is an advantage of the process according to the invention that the solvent boils at a relatively high temperature and, thus, the solvent may be recycled by distilling the hydrolysed dissolved feedstock degradation products from the solvent. In conventional organosolv processes, the solvent boils at a lower temperature and, thus, the solvent needs to be distilled from the feed degradation products, which requires a larger distillation capacity.

EXAMPLES

The process according to the invention will be further illustrated by means of the following non-limiting examples.

Examples 1 to 5

Approximately 4 grams of dried birchwood (Betula ssp.) sawdust having a cellulose content of 49 wt % was weighted into the reactor tube of a plug flow reactor. Solvent was continuously supplied to and discharged from the reactor tube at a liquid hourly velocity of 8 litre per kg wood per hour. The reactor pressure was kept at 4 bar (absolute). During the first hour, the solvent was heated from room temperature to the reaction temperature, then the temperature was maintained for one hour at the reaction temperature and then the solvent was cooled from reaction temperature to room temperature. After cooling, the solvent supply was stopped and acetone was supplied to the reactor to remove remaining liquid from the solid residue. The acetone-washed residue was dried by purging it overnight with nitrogen, removed from the reactor, and weighed.

During all experiments, a dark liquid fraction was discharged from the reactor tube and the acetone-washed residue obtained, i.e. the solid cellulosic fraction, had a considerably lighter colour than the feed material.

In the Table, the composition of the solvent, the reaction temperature, the amount of residue as percentage of the weight of the feed material and the cellulose content of the residue are shown for EXAMPLES 1 to 5. The cellulose content was determined by hydrolysing all polysaccharides followed by identification and quantification of sugars by means of gas chromatography according to TAPPI method T 249 cm-00. TABLE EXAMPLES 1 to 5 cellulose T residue content EXAMPLE Solvent (° C.) (wt %) residue 1 10.0 wt % gVL* 100 47 n.d.** 90.0 wt % formic acid 2 90 wt % gVL 150 43 n.d.** 10 wt % oxalic acid 3 97.1 wt % gVL 175 34 80 2.5 wt % phosphoric acid 0.4 wt % water 4 89.2 wt % gVL 175 65 63 10.8 wt % formic acid 5 78.6 wt % gVL 175 57 72 20.7 wt % ethylene glycol 0.6 wt % phosphoric acid 0.1 wt % water *gVL: gamma valerolactone **n.d.: not determined 

1. A process for organosolv pulping, wherein solid lignocellulosic feed material is heated at a temperature in the range of from 50 to 210° C. in a solvent to obtain a solid cellulosic fraction comprising at least 50 wt % of the cellulose present in the feed material and a liquid fraction, wherein the solvent comprises at least 10 wt % of a compound according to general molecular formula

wherein R₁ to R₆ each represent, independently, a hydrogen atom or an organic group connected with a carbon atom to the lactone group.
 2. A process according to claim 1, wherein the solid cellulosic fraction comprises at least 60 wt % of the cellulose present in the feed material.
 3. A process according to claim 1, wherein the cellulose in the solid cellulosic fraction obtained has an average degree of polymerisation of at least
 300. 4. A process according to claim 1, wherein the feed material is heated at a temperature in the range of from 100 to 200° C.
 5. A process according to claim 1, wherein the solvent comprises at least 20 wt % of the compound according to general molecular formula (1).
 6. A process according to claim 1, wherein the solvent comprises an acid catalyst.
 7. A process according to claim 6, wherein the acid catalyst is a strong mineral acid having a pKa below 2.5.
 8. A process according to claim 6, wherein the acid catalyst is a strong organic acid having a pKa below 2.5.
 9. A process according to claim 6, wherein the acid catalyst is present in a concentration of at most 5% by weight of the solvent.
 10. A process according to claim 1, wherein the feed material is heated at a temperature in the range of from 100 to 210° C. in a solvent that is free of mineral acid.
 11. A process according to claim 1, wherein the feed material is heated in the solvent at a pressure in the range of from 1 to 10 bar (absolute).
 12. A process according to claim 1, wherein R₁, R₂, R₃ and R₄ each are a hydrogen atom.
 13. A process according to claim 12, wherein R₅ is a methyl group.
 14. A process according to claim 13, wherein the compound according to general molecular formula (1) is gamma valerolactone (R₆ is a hydrogen atom), 2-methyl-5-oxotetrahydrofuran-2-carboxylic acid (R₆ is a carboxyl group), a compound having a molecular structure according to any one of molecular formulas (2) to (5):

or an ester of a compound having a molecular structure according to molecular formula (2) or (3).
 15. A process according to claim 1, wherein the compound having a gamma lactone group has a molecular structure according to molecular formula (6) or (7):

or is an ester of a compound having a molecular structure according to molecular formula (6) or (7).
 16. Use of a compound according to general molecular formula (1) in a solvent for organosolv pulping.
 17. A process according to claim 1, wherein the solid cellulosic fraction comprises at least 80 wt % of the cellulose present in the feed material.
 18. A process according to claim 2, wherein the cellulose in the solid cellulosic fraction obtained has an average degree of polymerisation of at least
 300. 19. A process according to claim 2, wherein the feed material is heated at a temperature in the range of from 100 to 200° C.
 20. A process according to claim 3, wherein the feed material is heated at a temperature in the range of from 100 to 200° C. 